Solid oxide fuel cells, called SOFCs, are known to have power generation efficiency superior to that of other types of fuel cells. Therefore, SOFCs have attracted attention due to their potential to build a new power generation system that can effectively use energy. The amount of development of SOFCs has been increasing recently.
SOFCs have a single cell structure in which a solid electrolyte has a fuel electrode on one side thereof and an air electrode on the other side. Yttria-stabilized zirconia (ZrO2)0.92(Y2O3)0.08; hereinafter abbreviated as “8YSZ”) is well known as an electrolyte material for forming a solid electrolyte. In addition, scandia-stabilized zirconia ((ZrO2)0.9Sc2O3)0.1; hereinafter referred to as “10ScSZ”) using scandia, which is a stabilizer of zirconia, is also known as an electrolyte material for solid electrolytes. 10ScSZ is characterized in that although the mechanical properties, such as three-point flexural strength and fracture toughness, of 10ScSZ are equivalent to those of 8YSZ, the electrical conductivity of 10ScSZ is nearly 3 times higher than that of 8YSZ.
It is known that 10ScSZ, which has a cubic crystalline form in a high temperature range, undergoes a phase transition from cubic to rhombohedral phase at around 550° C. The phase transition problematically results in a rapid drop in the electrical conductivity of 10ScSZ. That is, 10ScSZ is considered to be a material whose electrical conductivity largely depends on the temperature. 10ScSZ has another problem in that the phase transition leads to a volume change, consequently facilitating the formation of fine cracks in the sintered body of 10ScSZ. To stabilize the crystalline state of 10ScSZ, for example, it is reportedly effective to dissolve elements, such as Ce, In, and Ga, in 10ScSZ to form a solid solution (for example, see NPL 1). The addition of such elements to 10ScSZ stabilizes the crystalline state of 10ScSZ, reducing the temperature dependence of electrical conductivity. In particular, Zr0.89Sc0.10Ce0.01Oα, which is obtained by dissolving Ce in scandia-stabilized zirconia, has already been commercialized.